Phased array antenna systems play a highly important role in many areas, and indeed an increasingly important role in military and space applications. They are extensibly utilized in communications, surveilance and guidance systems. With the trend toward higher frequencies and the use of a large number of monolithic microwave and millimeter circuit components, the available space for signal distribution becomes a serious problem in many applications. For example, against this background the use of fiber optic cables or links to distribute the multitude of control signals represents an advantageous alternative to the use of conventional coaxial cables, waveguide structures or the like. Further, the use of optical links also provides an opportunity to utilize a novel beam steering technique as set forth in the description of this invention. Fiber optic lengths are also advantageous because of their relative immunity to interference (EMI and EMP), their excellent crosstalk isolation, and their small size and weight.
The concept of an optically controlled phased array antenna is known in the communications field. Such a system consists basically of fiber optical distribution feed system and an optical processing system responsible for the beam steering or scanning. The antenna array typically consists of up to 10,000 or more independent distributed transmit/receive (T/R) modules fabricated by monolithic microwave or millimeter wave techniques (MMIC). The main function of the fiber optic distribution system is to transmit the control signals for frequency and phase reference for synchronization of the T/R modules and to achieve the desired beam scanning. The replacement of coaxial cables or waveguides by the fiber optic network also provides a larger band width and smaller transmission losses compared to conventional systems, particularly at higher frequencies.
The problem of beam scanning is one that has received a great deal of attention in the prior art, but due to its complexity and the high frequencies involved it remains a threshold area where great improvement is desired. One of the features of this invention is the utilization of the properties of optical fibers for introducing phase delays in a controlled pattern so as to achieve desired beam steering. It is known in the prior art that optical fibers can be repeatedly stretched by up to about 7% of their length. This characteristic is utilized in this invention as a means of introducing controlled phase delays into the optical lengths. It is noted that researchers working with fiber optic sensors have used piezo-electric crystals to stretch fibers to balance the arms of interferometers K. In this invention, the use of piezo-electric crystals or equivalent electro-mechanical means is made to stretch the fibers and deliberately introduce time delays into the overall system, as discussed in detail hereinbelow.